1. Technical Field
The invention relates to a pigment dispersed liquid, a production method for the pigment dispersed liquid, and a light curable ink composition using the pigment dispersed liquid; and more particularly to a pigment dispersed liquid in which a monomer does not perform a polymerization during a dispersion treatment even if a polymerization inhibitor or an polymerization-inhibiting organic solvent is not used; a production method for that pigment dispersed liquid and the light curable ink composition containing that pigment dispersed liquid.
2. Related Art
When an inkjet pigment ink composition is prepared, dispersion treatment is generally performed on the pigment by using a disperser so that the pigment is stably dispersed in the ink composition. In a light curable pigment ink composition, a polymerizable compound is used as an ink solvent, but it is very difficult to perform dispersion treatment on both the pigment and the monomer together. The reason for that is that the monomer initiates polymerization in response to heat generated during the dispersion treatment or mechanochemically generated radicals, and that results in undesired phenomena such as increase in viscosity or gelatinization.
There is a method for removing that inconvenience by adding a polymerization inhibitor to control the polymerization of the monomer. For example, JP-A-2003-127518 discloses that ink storage stability, bleed resistance, stability (rubbing resistance etc.) after printing, and smoothness are improved in an ink composition containing 100-10,000 ppm polymerization inhibitor, deformation of a recording medium is controlled, and so a good image can be obtained and a stable and highly detailed image can be printed, irrespective of the type of recording medium (particularly media having no ink absorbability). As another example, JP-A-2003-306622 discloses that an active ray-curable ink containing (a) a pigment dispersed liquid containing 20-95 mass % pigment, (b) a polymerizable compound, (c) a polymerization initiator, and a polymerization inhibitor, the ink having 6-500 mPa·s viscosity at 25° C. and 0.01-2.5 mass % moisture content measured using Karl Fischer method has low viscosity and excellent sensitivity, outgoing stability, heat cycle resistance, and ink storage stability.
However, in the above techniques, the light curable pigment ink composition invariably contains a polymerization inhibitor, which inhibits the curing reaction of the light curable pigment ink composition. Therefore, when the light curable pigment ink composition is prepared, dispersion treatment is generally performed by using an polymerization-inhibiting organic solvent, and a monomer and photopolymerization initiator are added afterward.
Meanwhile, in that method, a drying process before the curing reaction is necessary for removing organic solvent existing in the ink that is not necessary for the curing reaction. Moreover, vaporizing an organic solvent into the air is not favorable in terms of the effect on the work environment, the cost of equipment such as a local exhauster and exhaust gas combustion apparatus, and the surroundings. A method of replacing organic solvent with monomer, e.g. a method in which organic solvent is removed by an evaporator or with a distillation operation, utilizing boiling point difference is also conceivable, but the number of production steps increases, along with the cost, and complete removal of the organic solvent is difficult.